1. Field of the Invention
The present invention is broadly concerned with improved mud additive compositions particularly formulated for treatment of wells characterized by the presence of downhole iron, in order to minimize hole enlargement when salt beds are encountered during the drilling process, and to reduce the need for fresh water in the control of salt deposition within the well. More particularly, the invention pertains to an aqueous composition including a first ingredient such as sodium or potassium ferrocyanide and mixtures thereof, and a second ingredient such as the trisodium salt of nitrilotriacetic acid or alkali metal citrates and mixtures thereof; the composition is added to recirculating drilling mud and lessens salt erosion even in the case of wells having high concentrations of iron therein. The composition may be added to circulation water to enhance solubilization of salt within the well.
2. Description of the Prior Art
During the course of oil well drilling operations, it sometimes occurs that a salt bed is encountered. When this happens, typical clay-based aqueous drilling mud can quickly erode the salt formation by dissolving salt, thereby creating a washed out annular zone. When this occurs, additional cement is needed to fill the hollowed out zone, thereby increasing drilling costs. The drill string may also become stuck as fluid velocity falls below cutting particle slip velocity in the widened zone, thereby causing particles to settle and bind the drill string. Furthermore, the hole enlargement greatly complicates formation evaluation efforts.
In response to these problems, it is conventional practice to add salt or saturated brine to the drilling mud through a hopper mixing system or fluid jetting system at the mud pit. In this way, annular erosion is lessened. However, this expedient is not entirely satisfactory, owing to the fact that downhole conditions of increasing temperature with depth enable brines that are salt-saturated at surface temperatures to dissolve additional salt in the well bore. As the resulting super-saturated brines are circulated to the surface and cooled, salt precipitates from the aqueous system. This precipitation can create a disposal problem in the pits and enables the brine to dissolve additional well bore salt during the next circulation.
Drilling mud additives have been commercialized in the past which serve to prevent dilution of the brine fraction of the recirculating drilling mud. One such amine-based additive is presently available, but costs about $22 per gallon. Sodium ferrocyanide is a very effective and relatively inexpensive treating agent, and is therefore preferred. However, sodium ferrocyanide has a significant drawback in that if the well and its products contain iron, the effectiveness of the treatment is drastically reduced, and a blue, scale-like amorphous substance is deposited on downhole equipment. This leads to downtime and loss of production, and even a need to replace downhole equipment.
During the course of oil or gas production, some wells may produce high chloride brines which tend to deposit salt on well equipment. This can adversely affect well production. One solution to this problem is to circulate fresh water through the well to dissolve the deposited salt. However, large volumes of fresh water may be required for this purpose.
There is accordingly a real and unsatisfied need in the art for an improved composition making use of an inexpensive ferrocyanide for brine control during drilling and well operation, while at the same time having the ability to effectively operate in downhole environments characterized by the presence of iron.